Re: [PATCH v2] btrfs: Fix memory barriers usage with device stats counters

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- Nikolay Borisov <nborisov@suse.com> wrote:
> Commit addc3fa74e5b ("Btrfs: Fix the problem that the dirty flag of dev stats is cleared")
> reworked the way device stats changes are tracked. A new atomic dev_stats_ccnt
> counter was introduced which is incremented every time any of the device stats
> counters are changed. This serves as a flag whether there are any pending stats
> changes. However, this patch only partially implemented the correct memory
> barriers necessary:
> 
> - It only ordered the stores to the counters but not the reads e.g. btrfs_run_dev_stats
> - It completely omitted any comments documenting the intended design and how
> the memory barriers pair with each-other
> 
> This patch provides the necessary comments as well as adds a missing smp_rmb in
> btrfs_run_dev_stats. Furthermore since dev_stats_cnt is only a snapshot at best
> there was no point in reading the counter twice - once in btrfs_dev_stats_dirty
> and then again when assigning stats_cnt. Just collapse both reads into 1.
> 
> Fixes: addc3fa74e5b ("Btrfs: Fix the problem that the dirty flag of dev stats is cleared")
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

> ---
> 
> V2: 
>  Reworded the comment in btrfs_run_dev_stats according to Mathieu Desnoyers'
>  feedback 
> 
>  fs/btrfs/volumes.c | 18 ++++++++++++++++--
>  fs/btrfs/volumes.h | 12 ++++++++++++
>  2 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 4de498817e8a..7f8ed3e08ece 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -7084,10 +7084,24 @@ int btrfs_run_dev_stats(struct btrfs_trans_handle *trans,
>  
>  	mutex_lock(&fs_devices->device_list_mutex);
>  	list_for_each_entry(device, &fs_devices->devices, dev_list) {
> -		if (!device->dev_stats_valid || !btrfs_dev_stats_dirty(device))
> +		stats_cnt = atomic_read(&device->dev_stats_ccnt);
> +		if (!device->dev_stats_valid || stats_cnt == 0)
>  			continue;
>  
> -		stats_cnt = atomic_read(&device->dev_stats_ccnt);
> +
> +		/*
> +		 * There is a LOAD-LOAD control dependency between the value of
> +		 * dev_stats_ccnt and updating the on-disk values which requires
> +		 * reading the in-memory counters. Such control dependencies
> +		 * require explicit read memory barriers.
> +		 *
> +		 * This memory barriers pairs with smp_mb__before_atomic in
> +		 * btrfs_dev_stat_inc/btrfs_dev_stat_set and with the full
> +		 * barrier implied by atomic_xchg in
> +		 * btrfs_dev_stats_read_and_reset
> +		 */
> +		smp_rmb();
> +
>  		ret = update_dev_stat_item(trans, fs_info, device);
>  		if (!ret)
>  			atomic_sub(stats_cnt, &device->dev_stats_ccnt);
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index 6108fdfec67f..c5dd48eb7b3d 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -498,6 +498,12 @@ static inline void btrfs_dev_stat_inc(struct btrfs_device *dev,
>  				      int index)
>  {
>  	atomic_inc(dev->dev_stat_values + index);
> +	/*
> +	 * This memory barrier orders stores updating statistics before stores
> +	 * updating dev_stats_ccnt.
> +	 *
> +	 * It pairs with smp_rmb() in btrfs_run_dev_stats().
> +	 */
>  	smp_mb__before_atomic();
>  	atomic_inc(&dev->dev_stats_ccnt);
>  }
> @@ -523,6 +529,12 @@ static inline void btrfs_dev_stat_set(struct btrfs_device *dev,
>  				      int index, unsigned long val)
>  {
>  	atomic_set(dev->dev_stat_values + index, val);
> +	/*
> +	 * This memory barrier orders stores updating statistics before stores
> +	 * updating dev_stats_ccnt.
> +	 *
> +	 * It pairs with smp_rmb() in btrfs_run_dev_stats().
> +	 */
>  	smp_mb__before_atomic();
>  	atomic_inc(&dev->dev_stats_ccnt);
>  }
> -- 
> 2.7.4
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com